1. Field of the Invention
The present invention is directed to a measuring instrument for intracardial acquisition of the blood oxygen saturation which can be used to control the frequency of stimulation pulses supplied to a patient by a heart pacemaker, and in particular to such a measuring instrument having a light emitter and a light receiver, wherein the light receiver receives light from the light transmitter reflected by the blood.
2. Description of the Prior Art
A measuring instrument is disclosed in German OS No. 31 52 963 having a light emitter and a light receiver contained in a measuring probe disposed in a patient's heart. The transmitter and receiver are connected in parallel to an evaluation circuit via two leads. The probe includes a light emitting diode and a phototransistor connected in parallel such that the conducting state current through the light emitting diode is superimposed with the current through the phototransistor caused by the influence of light. If the measuring probe is operated with a constant current or a constant voltage, the light reflected by the blood causes a current change in the phototransistor dependent on the oxygen saturation of the blood, which in turn causes a change in the current or voltage at the measuring probe which is "seen" by the evaluation circuit. The change in voltage or current generated by light reflection is identified by comparing the measured signal to a reference signal.
The reference signal is generated using a pulse, such as a voltage pulse, having the same magnitude but an opposite operational sign, as is used for the useful signal measurement. This pulse is conducted through a diode connected with polarity opposite to that of the light emitting diode. The operating characteristic of the diode in the reference circuit and the characteristics of the light emitting diode are preferably identical.
In this conventional measuring instrument, therefore, only two electrical leads are necessary to make both the useful signal measurement and the reference measurement. This is of particular significance because such lead must be accomodated in a catheter, which preferably has the smallest possible diameter and high flexibility. Moreover, the presence of each additional lead within the catheter increases the possibility of a malfunction.
A disadvantage of this conventional instrument, however, is the necessity of re-polarization of the measured signal, i.e., the necessity of generating a signal for the reference measurement which is of opposite polarity to the signal used for the useful signal measurement. Given the standard format of the voltage supply in heart pacemakers, wherein one pole of the supply voltage is permanently connected to the housing, this repolarization capability involves a significant circuit outlay. Another disadvantage is that the same current is used for the reference measurement as for the useful signal measurement.